Experimental and theoretical study on pullout resistance of grouted soil nails

Abstract

Soil nails have been widely used for stabilization of slopes and earth retaining structures in many countries and regions. In Hong Kong, grouted soil nails have been used in most slope stabilization works since the late 1980s. Soil nail pullout resistance is a key parameter in the design of soil nailing. The design of a soil nailed system for slope stabilization will be improved, if the pullout resistance can be more accurately estimated. Nowadays, researchers and engineers have acknowledged that many factors influence the pullout resistance of soil nails, such factors include installation method, overburden pressure, grouting pressure (for grouted nails), roughness of nail surface, degree of saturation of the soil, soil dilation, bending of the soil nail. This research has therefore commenced to study the pullout resistance of grouted soil nails. Firstly a simple mathematical model for the interaction analysis of a soil nail and the surrounding soil has been developed. This model takes into account some key factors, which are constrained soil dilation, soil nail bending, vertical pressure, and non-linear subgrade reaction stiffness. The lateral subgrade reaction between the soil and the soil nail is assumed to obey a hyperbolic relation. Reported test data in the literature are used to verify the present model. Good agreement is found from the comparisons in two case studies. The analyses show that the contribution of the soil nail bending to the pullout resistance is of secondary importance as the tension failure is dominant in a soil-nailed slope. Parametric study indicates that the soil nail pullout resistance increases to a limit value as the soil nail bending stiffiiess approaches to infinity. Secondly, a series of laboratory soil nail pullout tests have been carried out to study the influences of both grouting pressure and overburden pressure on the soil nail pullout interface shear resistance. The pullout tests were conduced in a completely decomposed granite (CDG) soil in a saturated condition. The test procedures simulated the real construction process of a soil nail, including the establishment of initial soil stress, drilling a hole with stress release, pressure grouting, soil saturation, and soil nail pullout. The pullout box was well instrumented. Typical test results are presented and discussed. From the data analysis, the effects of both grouting pressure and overburden pressure on the soil nail pullout resistance are investigated. It is found that both overburden pressure and grouting pressure have influences on soil nail pullout resistance, and their influences are interactional. The soil nail pullout resistance is hardly or slightly dependent on the overburden pressure when the grouting pressure is low, but increases with the overburden pressure when the grouting pressure is higher. Based on the test results, a new empirical liner equation is proposed for the determination of soil nail pullout resistance considering both grouting pressure and overburden pressure. Finally, a three-dimensional (3D) finite element model has been established for modelling the laboratory soil nail pullout tests carried out in The Hong Kong Polytechnic University. The model simulates all the procedures of the pullout tests for the cases of both unsaturated soil without pressure grouting and saturated soil with pressure grouting. The model is verified by the comparisons between the results from the numerical modelling and the pullout test data. It is found that the present 3D FE model is capable of capturing the main features of the soil stress variations and pullout behaviour during the soil nail pullout tests in both unsaturated and saturated soils. The FE modelling shows that the stress distribution in the soil nail axial direction is non-uniform at large pullout displacement in the unsaturated soil condition, but basically uniform in the saturated condition.